THE SILURIAN PERIOD. 



393 



formations. In Wisconsin, for example, remnants (outliers, a, b, c) 

 of the Niagara are found far beyond the main body (S, Fig. 183) of 



Fig. 183. — Map showing the surface distribution of Silurian, Ordovician, and other 

 formations in southern Wisconsin, northern Illinois, and eastern Iowa. € = 

 Cambrian; = Ordovician ; £ = Niagara, and a, 6, c are outliers of S; M— 

 Mississippi an ; and C = Carboniferous. 



the formation as it now exists. These remnants fix at least a minimum 

 limit to the original extension of the formation. The strati graphical 

 relations are indicated by the accompanying section (Fig. 184), which 



Fig. 184. — Section showing the structure of the Silurian and other beds in Wisconsm. 

 €= Cambrian; = Ordovician ; S= Silurian; and D = Devonian. The black 

 belt at the surface = glacial drift, etc. (Wis. Geol. Surv.) 



at once suggests the basis for the estimate. Farther north the Silurian 

 formations appear to have been completely removed from great areas 

 which they once covered. 



The map (Fig. 185) of a part of New York shows the relations of 

 the outcrops of the several Silurian formations in the western part of 

 that State. Their former northward extension can only be conjectured, 

 but it was probably considerable. Fig. 186 shows the stratigraphic 

 relations of the beds shown on the map, the section being from north to 

 south. Figs. 184 and 186 fairly illustrate the stratigraphic relations of 

 the Silurian in the interior generally. Their stratigraphic relations in 

 the Aooalachian mountains are illustrated by Figs. 138 and 139, where 



